Electric engine start systems utilize a motor drive (a switching-mode power conversion system) to control a motor/starter and employ an active rectifier as a front-end power converter to interface with a power supply. The active rectifier draws near sinusoidal currents from the power supply to meet specification requirements on input current harmonics. Both the active rectifier and the motor drive inverter employ fast switching IGBT devices controlled using a pulse-width modulation (PWM) technique. The high frequency voltage components of PWM converter (the active rectifier) and inverter cause high frequency leakage currents and conducted EMI in the power lines and ground system.
The conducted EMI can be classified into two-types: common-mode (CM) and differential-mode (DM) according to their circulation paths. The noise flowing within the power-feeding paths is called DM noise, and the noise flowing between the power conversion system and the ground is called CM noise. Typical EMI filters consist of an input section connected between the active rectifier and the power source, and an output section connected between the drive inverter and the motor. Both sections include DM and CM inductors, capacitors and resistors. The output section reduces high-frequency leakage currents due to parasitic motor winding capacitances to ground, as well as bearing currents, and shaft voltage. The radiated emissions are controlled by shielding drive inverter power lines to the motor.
In some systems the motor drive inverter operates at higher output current in comparison to the active rectifier input current that results in a larger weight of EMI filter output section. As an alternative to the excessive weight of EMI filter output section, some systems that operate a motor drive (i.e. engine start) at low duty cycle and utilize shielded output cable to the motor have implemented additional common mode filtering in the EMI filter input section to address additional CM noise contributed by the motor windings capacitive coupling to the ground.
For safety reasons common mode filters do not use an increased total capacitance to ground for filtering. Instead, common mode filters use an increased inductance of at least one common mode choke in a multi-stage common mode filter in order to increase the insertion loss of the common mode filter. This construction makes the common mode filter bulky.